Field of the Invention
The present invention relates to a machine tool configured to chamfer corners of a workpiece and a chamfering method.
Description of the Related Art
Conventionally, corners of a workpiece are chamfered by means of a machine tool comprising a spindle and a table to which a workpiece is secured. The spindle is secured to a certain linear movement axis, the table is moved along two or more linear movement axes, and the spindle and the table are controlled by a numerical controller. As shown in FIGS. 7A to 7D, the corner chamfering is performed based on the motion of two linear movement axes (X- and Y-axes) perpendicular to the axial direction (Z-axis direction) of a tool with a linear movement axis in the axial direction fixed.
(I) First, machining conditions for the case where a workpiece 3 is chamfered to a depth of 1.0 mm by means of a 45°-chamfering tool 2 with a maximum diameter of φ6.0 mm are set, as shown in FIG. 7A.
(II) Then, the workpiece 3 is chamfered in the set machining conditions, as shown in FIGS. 7B and 7C.
(III) That part of the cutting edge of the chamfering tool 2 used for actual cutting at three chamfered portions 5, 6 and 7 of the workpiece 3 shown in FIG. 7B is a portion with a diameter of φ3.0 mm.
In the machining method described above, the same part (chamfering area 8 shown in FIG. 7D) of the cutting edge of the chamfering tool 2 is always used for chamfering, so that it is solely subject to progressive wear, as shown in FIG. 7D. Even if the cutting edge of the chamfering tool 2 used still includes a usable part when it reaches the end of its useful life, therefore, the tool 2 cannot be used with a machining program for the workpiece 3. Accordingly, the program or tool length and diameter compensation values must be modified, so that high-efficiency automation cannot be achieved for mass-produced parts and the like.
Thus, according to the machining method described above, an expert programmer must create complicated programs, and a change of the lifespan value due to a change of the shape, tool material, or coating requires a substantial program change. Although this method ensures an improvement in tool lifespan, it lacks in versatility.
In some cases, therefore, a method of tool lifespan improvement may be adopted in which the tool length and diameter are changed in a program, depending on a set lifespan frequency, so that the position of contact of the cutting edge with the workpiece 3 can be shifted to increase the number of times of usage for each chamfering tool, as shown in FIGS. 8A to 8C, 9 and 10.
If the machining frequency is not higher than “<1> lifespan frequency”, the diameter of the chamfering tool 2 at the chamfering area 8 is set to φ5.0 mm, as shown in FIG. 8A. If the machining frequency is not higher than “<2> lifespan frequency”, the diameter of the chamfering tool 2 at the chamfering area 8 is set to φ3.0 mm, as shown in FIG. 8B.
FIG. 8C illustrates chamfering areas 8 with tool diameters of φ5.0 mm and φ3.0 mm.
As shown in FIG. 9,
(1) a calculation formula (#600=#600+1) for accumulating the machining frequency,
(2) a calculation formula for comparing a preset tool lifespan frequency and the machining frequency and modifying the tool length and diameter compensation values in the machining program (conditional branches <1> and <2>), and
(3) a calculation formula for resetting the rotational frequency of the spindle on which the tool is mounted and a feed rate, based on the tool diameter modified to maintain a constant machining peripheral speed so that the machining frequency quality cannot be changed by the modification of the tool diameter compensation value
are inserted into a machining program (O0001) for chamfering (N001 and N002). In this way, the number of times of usage of the chamfering tool can be increased to improve the lifespan of each tool. The machining peripheral speed is a cutting speed (m/min) at the outer diameter of the tool in contact with the workpiece to be machined.
According to the machining method shown in FIGS. 8A to 8C, 9 and 10, however, an expert programmer's programming is required, and machining conditions are modified to stabilize the machining quality, so that the machining time is not fixed. Further, a change of the shape or tool material requires a program change by an expert programmer. Thus, this method lacks in versatility.
On the other hand, Japanese Patent Application Laid-Open No. 5-8148 discloses a technique to improve tool lifespan by using the entire area of a circular-arc-shaped cutting edge of a tool in a machine tool comprising at least three controllable axes. The entire cutting edge can be used by changing the posture of the tool relative to a workpiece. However, this technique is only applicable to machining using tools with a circular-arc-shaped cutting edge, and moreover, to a machine tool with a rotating shaft capable of changing the tool posture relative to the workpiece during the machining.